(1) Field of the Invention
The present invention relates to a stored light intensity measurement device for measuring afterglow intensity of a light storing sign.
(2) Description of the Related art
When the earthquake or the like causes a power failure at night, a self-generation of electricity type or battery-powered type pilot display light is to light in an underground passage, in a passage in a building, or in the subway yard. However, there are occurrences when the self-generation of electricity does not always work at the earthquake, and the battery-powered pilot display light does not light at the emergency due to failing the maintenance of the battery of the pilot display light.
Therefore, Japanese government promotes an installation of light storing signs, which keep the luminescence in a specific period after the power failure without supplying energy from outside, on a wall and a riser of stairs in the above-mentioned sites. The light storing sign is a sign board made up of synthetic resin, ceramic, or glass, and of which surface is processed by the light storing material and is given a safety sign design with coloring materials.
The light storing material can absorb light such as sunlight or fluorescent light, and store the energy, and release the stored energy as visible light. The light storing sign using the light storing material is visible due to its phosphorescence in dark environment, and it is also visible even when ambient darkness occurs due to the power failure.
The phosphorescence of the light storing sign fades away and becomes invisible with the lapse of time. Recently, in case of strontium aluminate or calcium aluminate that are used as the light storing material, the visible state will last for approximate 10 hours in total darkness. In case of zinc sulfide, the visible state will last only for a few hours.
In order that the sign is useful in darkness, the sign should have necessary luminance after a predetermined time lapsed. Japanese Industrial Standards Z9107 regulates the light storing sign for communicating or displaying for caution, indication or information for evacuation. That is to say, the luminance of the phosphorescence requires 24 mcd/m2 and more after 20 minutes from the light interception.
In order to measure the afterglow intensity of the light storing sign, the luminance meter like the spot-meter is useful, as disclosed in Japanese Patent Unexamined-Publication No. 07-011250A. For instance, the light storing sign is placed in total darkness for hours before the measurement is performed, and then a predetermined illuminance light is irradiated thereon to store the energy of light. After that, the light storing sign is moved again in darkness, and the afterglow intensity of the light storing sign is measured by the luminance meter.